Random number generating device

ABSTRACT

The objective is to provide a random number generating device having a smaller circuit size and a smaller value of output bias. The random number generating device includes a pair of first and second current paths arranged in parallel with each other, and a pair of first and second fine particles, which can mutually exchange charges, and are located in the vicinity of the first and second current paths.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 10/373,874filed Feb. 27, 2003, and based upon and claims the benefit of priorityfrom the prior Japanese Patent Application No. 2002-54153, filed on Feb.28, 2002, the entire contents of each of which are incorporated byreference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a random number generating device.

2. Related Background Art

As a result of the improvement in information communication networks,such as Internet, commercial transactions such as bank payments arecarried out through information communication networks. As suchopportunities increase, a demand for higher security arises, resultingin that various kinds of cryptosystems are developed.

In such cryptosystems, it is necessary to generate high-quality randomnumbers in order to improve the security level. The term “high-quality”means that there is no periodicity in random numbers, that it isimpossible to predict the random numbers, etc.

Conventionally, random numbers have been generated by the use ofcalculating software such as a shift register. However, since the randomnumber generated in such a manner are pseudo-random numbers, if thereare considerably many numbers, a periodicity appears, thereby decreasingthe security level.

In order to generate high-quality true random numbers, there are methodsin which physical random numbers are generated based on physicalphenomena such as thermal noises. Such physical random numbers are truerandom numbers in principle. Therefore, these methods are ultimatemethods of generating random numbers.

A device generating random numbers by amplifying thermal noise signalsof a diode is proposed in the above-described methods. In this device, avery subtle thermal noise of the diode is amplified by using variouskinds of amplifiers. Accordingly, in order to generate high-qualityrandom numbers, the circuit size inevitably becomes large.

Further, in this device, random numbers should be generated based on thediode current/voltage characteristics. Accordingly, the outputs areoften biased.

Moreover, as the processing speed of semiconductor chips is increased,the speed at which random numbers are generated should also beincreased.

Thus, conventional random number generating devices have problems inthat the circuit size there of is large, and the outputs thereof arebiased.

SUMMARY OF THE INVENTION

The present invention is proposed in consideration of the aboveproblems, and the object of the present invention is to provide a randomnumber generating device having a smaller circuit size, and a smallervalue of output bias.

A random number generating device according to the first aspect of thepresent invention includes: a pair of first and second current pathsarranged in parallel with each other; and a pair of first and secondfine particles which are located in the vicinity of said first andsecond current paths, and which can mutually exchange charges, at leastone of the first and second fine particles being electrically connectedto one of the first and second current paths.

It is preferable that the first and second fine particles are locatedbetween the first and second current paths.

Further, it is preferable that the energy level including the chargingenergy of a charge in the first and second fine particles is discrete.

Moreover, it is preferable that the diameter of the first and secondfine particles is 100 nm or less.

In addition, it is preferable that an odd number pairs of first andsecond fine particles are provided along the direction of the currentsflowing through the first and second current paths.

A random number generating device according to the second aspect of thepresent invention includes: a pair of first and second current pathsarranged in parallel with each other; an insulating layer formed betweenthe first and second current paths; and a nanoparticle having a diameterof 1 μm or less located in the insulating layer, at least one of acapacitance of a first capacitor including the first current path, theinsulating layer, and the nanoparticle and a capacitance of a secondcapacitor including the second current path, the insulating layer andthe nanoparticle being 1 nF or less.

It is preferable that plasma oscillation occurs in the nanoparticle, andthe plasma frequency of the nanoparticle is 1/10 or more of thefrequency of fluctuations of the currents flowing through said first andsecond current paths, and ten times the frequency of fluctuations of thecurrents flowing through said first and second current paths or less.

Further, the insulating layer may include a first insulating layerlocated between the first current path and the nanoparticle, and asecond insulating layer located between the second current paths and thenanoparticle.

It is preferable that there are an odd number of nanoparticles.

A random number generating device according to the third aspect of thepresent invention includes: a pair of first and second current pathsarranged in parallel with each other; and a layer including a pluralityof trap levels formed between the first and second current paths, thetrap levels being able to mutually exchange charges, the trap levelsbeing electrically coupled with at least one of the first and secondcurrent paths.

The term “parallel” does not necessarily mean that the current paths arecollimated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram of a random number generating deviceaccording to the first embodiment of the present invention.

FIG. 2 is another conceptual diagram of a random number generatingdevice according to the first embodiment of the present invention.

FIG. 3 shows the potential of a pair of quantum dots.

FIG. 4 shows that an odd number pairs of quantum dots each having afirst nanoparticle 3 and a second nanoparticle 4 are arranged between afirst current path 1 and a second current path 2.

FIGS. 5A to 5C show a random number generating device according to thesecond embodiment of the present invention. FIG. 5A is a section view;FIG. 5B is a section view; and FIG. 5C is a perspective view.

FIGS. 6A and 6B show the structure of a random number generating deviceaccording to the third embodiment of the present invention.

FIG. 7 is a section view of a random number generating device accordingto the fourth embodiment of the present invention.

FIGS. 8A to 8K show the random number generating device according to thefirst example of the present invention. FIG. 8A is a section view; FIGS.8B to 8F and 8H are section views showing the main fabricating steps;FIG. 8G are a top view; FIG. 8I is a perspective view; FIG. 8J is asection view of a modification; and FIG. 8K is a perspective view of themodification.

FIGS. 9A to 9J show the random number generating device according to thesecond example of the present invention. FIG. 9A is a section view;FIGS. 9C to 9E are section views showing the main fabricating steps;FIGS. 9B and 9F are top views; FIG. 9G is a perspective view; and FIGS.9H to 9J are section views of a modification.

FIGS. 10A to 10G show the random number generating device according tothe third example of the present invention. FIG. 10A is a section view;FIGS. 10B to 10F are section views showing the main fabricating steps;and FIG. 10G is a top view.

FIG. 11 is a section view of a random number generating device accordingto the fourth example of the present invention.

FIG. 12 is a section view of a random number generating device accordingto the fifth example of the present invention.

FIG. 13 is a section view of a random number generating device accordingto the sixth example of the present invention.

FIG. 14 is a section view of a random number generating device accordingto the seventh example of the present invention.

FIG. 15 is a section view of a random number generating device accordingto the eighth example of the present invention.

FIGS. 16A to 16C are section views of random number generating devicesaccording to other examples of the present invention.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings. It should be noted that thepresent invention is not limited to the following embodiments, but canbe modified in various ways.

First Embodiment

FIG. 1 is a conceptual view of a random number generating deviceaccording to the first embodiment of the present invention.

As shown in FIG. 1, the random number generating device includes a pairof first current path 1 and second current path 2, which are placed inparallel with each other, and a pair of first nanoparticle 3 and secondnanoparticle 4, which are capable of exchanging electric charges, andare located between the first current path 1 and the second current path2.

The size of the first nanoparticle 3 and the second nanoparticle 4 issufficiently small to enjoy the effects of the wave function of charges.For example, the diameter of the nanoparticles is 100 nm or less. Suchnanoparticles are called “quantum dots.” In a quantum dot, the energylevel including charging energy of a charge is discrete.

Further, the first nanoparticle 3 and the second nanoparticle 4 areelectrically coupled via a tunnel barrier 6 having a capacitance ofC_(B) and a resistance of R_(B), end charges can be transferred betweenthem.

Moreover, the first nanoparticle 3 and the first current path 1 areelectrically coupled via a barrier 5 having a capacitance of C_(A) and aresistance of R_(A); and the second nanoparticle 4 and the secondcurrent path 2 are electrically coupled via a barrier 7 having acapacitance of C_(C) and a resistance of R_(C).

It is assumed that a pair of quantum dots 23, which is formed bycoupling the first nanoparticle 3 and the second nanoparticle 4, islocally not within the conditions of electrical neutrality. That is, itis assumed that an extra charge 8 is injected into the pair of quantumdots 23 from the first current path 1 or the second current path 2, orthat the pair of quantum dots 23 is polarized. This state can beachieved if the formulas R_(A)>>R_(B) and R_(C)>>R_(B) hold, and chargesare transferred more easily within the pair of quantum dots 23, i.e.,between the first nanoparticle 3 and the second nanoparticle 4, thanbetween the current paths and the pair of quantum dots 23.

This random number generating device is achieved based on the fact thatwhen the diameters of the first nanoparticle 3 and the secondnanoparticle 4 are on the order of nanometers, the probability that acharge such as an electron exists within the first nanoparticle 3 or thesecond nanoparticle 4 varies with the physical uncertainty. A physicalrandom number can be generated by observing the effect of the chargedistribution, which shifts with the physical uncertainty, on thecurrents flowing through the first current path 1 and the second currentpath 2.

For example, it is assumed that the capacitance of a miniaturizedstructure is C˜∈₀S/d, where S is the maximum section area of theminiaturized structure viewed from a direction parallel to the twocurrent paths, d is the minimum length between two structures (quantumdots), and ∈₀ is the dielectric constant.

Assuming that two silicon nanoparticles having a diameter of 10 nm,which serve as the first nanoparticle 3 and the second nanoparticle 4described above, are located with a distance of 2.5 nm, the capacitanceis about 2.2 aF (10⁻¹⁸ F). In this case, the charging energy perelectron is about 36 meV.

The charging energy per electron of 36 meV is sufficiently higher thanthe thermal fluctuation energy of 25.8 meV at the room temperature of300 K. Accordingly, in the pair of quantum dots 23, the changes incharge distribution can be observed at the room temperature. Thisphenomenon is generally called “Coulomb Blockade.”

Assuming that the resistance existing when a charge is transferredbetween localized levels is R, time Δt_(d) in which a charge isstabilized in a localized level is estimated to be Δt_(d)˜1/(C·R).

For example, if the tunnel resistance R_(B) between the firstnanoparticle 3 and the second nanoparticle 4 is 10⁶Ω and the CoulombBlockade effect can be observed, Δt_(d) is about 4.5×10⁻¹¹ seconds.Here, the resistance R with which the Coulomb blockade effect can beobserved is estimated to be 25.8 kΩ or more, and the capacitance C isestimated to be 2.2 aF.

In FIG. 1, if a current flows through, e.g., the first current path 1,the charge distribution within the first nanoparticle 3 changes due tothe Coulomb repulsion from the electrons of the current flowing throughthe first current path 1, so that an electron 8 is rearranged so as togo away from the current flowing through the first current path 1.

It is assumed that the current path is a source-drain path between asilicon FET. Assuming that the mobility between the source and the drainis μ, and the electric field applied between the source and the drain isE, the velocity v of a charge within the current flowing through thepath between the source and the drain is given by v=μE.

Assuming that the diameter of the first nanoparticle 3 and the secondnanoparticle 4 is I_(d), time Δt_(I) in which a charge flowing troughthe first current path 1 passes in the vicinity of the firstnanoparticle 3 and the second nanoparticle 4 is estimated to beΔt_(I)˜I_(d)/V=I_(d)/(μE). If one of Δt_(d) and Δt_(I) is less than tentimes the other, the current detects the rearrangement of electronwithin the pair of quantum dots. That is, the ratio between Δt_(d) andΔt_(I) is 1/10 or more and 10 or less. In addition, this can be achievedif one of the two values is 1,000 times the other or less. That is, theratio between Δt_(d) and Δt_(I) is 1/1000 or more and 1000 or less.

If it is assumed, for example, that the mobility p is 1000 cm²/(Vs), andthe electric filed E is 10⁶ cm/s, the velocity of electron is 10¹⁵ nm/s.Accordingly, if the diameter of the first nanoparticle 3 is 10 nm, timeΔt_(I) in which an electron passes in the vicinity of the firstnanoparticle 3 is 10⁻¹³ seconds, and there is a strong interactionbetween the charge distribution within the first nanoparticle 3 and thecurrent flowing through the first current path 1.

At this time, the charge distribution within the first nanoparticle 3fluctuates due to the current flowing through the first current path 1,and the fluctuations in charge distribution is reflected back in thecurrent flowing through the first current path 1, so that the currenttemporally fluctuates. Here, the second current path 2 is located in thevicinity of the second nanoparticle 4 and the values of the currentsflowing through the first current path 1 and the second current path 2are adjusted to be substantially on the same order. In order to adjustthe values of the currents so as to be substantially on the same order,the first current path 1 and the second current path 2 are designed soas to be formed of the same material, and to have substantially the samesection area, and the same voltage is applied to both of them.

Since the state of charge within the pair of quantum dots 23, i.e.,within the first nanoparticle 3 and the second nanoparticle 4 variesdepending on the situations, each time it is measured, the differencebetween the currents flowing through the first current path and thesecond current path is uniformly distributed in the plus or minus regionnear zero.

Thus, as shown in FIG. 2, if the difference between the currents flowingthrough the first current path 1 and the second current path 2 istransferred to a differential amplifier 90, it is possible to obtainrandom numbers as current values in consideration of the state beforethe currents flow through the first current path 1 and the secondcurrent path 2.

FIG. 2 shows, in a simplified manner, the differential amplifier 90,which amplifies the difference between the current flowing through thefirst current path 1 and the current flowing through the second currentpath 2, and outputs the result from an output terminal 91.

In the manner as described above, physical random numbers can begenerated by a physical phenomenon.

In this physical phenomenon, since the current values fluctuate in theplus or minus region in the vicinity of zero, the deviation between “0”and “1”, which often occurs in a random number generating device foramplifying thermal noises from, e.g., a diode, is inherently unlikely tooccur.

Further, as is understood from the calculation of Δt_(d) describedabove, it is possible to achieve the operation speed on the order ofpicoseconds (10⁻¹² seconds) or less just by adjusting the distancebetween the first nanoparticle 3 and the second nanoparticle 4.

In the random number generating device according to this embodiment,changes in quantum wave function has a strong influence. The act toobserve the charge distribution of the pair of quantum dots 23, i.e.,the first nanoparticle 3 and the second nanoparticle 4, mainly bringsabout the convergence of the wave packet of the wave function.

Accordingly, random numbers generated by the random number generatingdevice according to this embodiment are physical random numbers usingquantum fluctuations, which are not predictable in a quantum manner.Therefore, even if the number of the pairs of nanoparticles increases,no periodicity appears.

FIG. 3 schematically shows the potential of the quantum dot pair 23.

The movement of charges in the quantum dot pair 23 is expressed by theSchroedinger equation (T. Tanamoto, Physical Review A Vol. 61, p022305(2000)).

Electrons come and go between the two quantum dots in a cycle dependingon the height and the thickness of the tunneling barrier between thequantum dots. In which of the two quantum dots (the first nanoparticle 3and the second nanoparticle 4) a charge exists cannot be predicted withcertainty by the observation using the first current path 1 or thesecond current path 2 placed nearby.

This is the nature of the quantum theory. What is predictable is thesquare of the amplitude of the wave function, i.e., the probability ofexistence of a charge in each of the two quantum dots. If the twoquantum dots have the same electro-magnetic potential structure, theprobability of the existence of a charge is the same for both twoquantum dots.

Accordingly, random numbers can be generated by observing which of thetwo quantum dots has a charge, and setting “0” when one of them has acharge, and “1” when the other has a charge. In this case, each quantumdot is not necessarily in the ground level, but it is possible to be inan excitation state due to the thermal effect.

FIG. 4 shows that an odd number pairs of quantum dots 23 each having afirst nanoparticle 3 and a second nanoparticle 4 are arranged betweenthe first current path 1 and the second current path 2. In each quantumdot pair, the probabilities of the appearance of the “0” current and the“1” current are about the same in average.

Even if the speed of a charge in a current flowing through the firstcurrent path 1 or the second current path 2 is slow, and the effect ofthe polarization in the quantum dot pair 23 is averaged, if there areodd number pairs of quantum dots 23, the first current path 1 and thesecond current path 2 has the effect of either “0” or “1” currentwithout fail.

If there are three or more pairs of quantum dots 23, the Coulomb forceacts thereon so that the distribution of charge is opposite in theadjacent two quantum dot pairs 23. Accordingly, it is possible tofurther inhibit the deviation of distribution in one direction in thequantum dots.

Further, even if any of the quantum dot pairs does not function, or onlyone of the quantum dots has the deviation of charge, the random numbergenerating device in this embodiment can function since it has aplurality of quantum dot pairs.

Second Embodiment

Next, a random number generating device according to the secondembodiment of the present invention will be described with reference toFIGS. 5A to 5C.

In this embodiment, the effect of the interaction between the temporalchanges in potential distribution of a capacitance network, the chargestate of which is not stable, and the temporal fluctuations in currentis mainly used.

As shown in FIG. 5A, the random number generating device includes a pairof first current path 1 and second current path 2 arranged in parallelwith each other, a microparticle 9 which has a diameter of 1 μm or lessand is located between the first current path 1 and the second currentpath 2, a first insulating layer 10 located between the microparticle 9and the first current path 1, and a second insulating layer 11 locatedbetween the microparticle 9 and the second current path 2.

With such a structure, at least one of a first capacitance constitutedby the first current path 1, the first insulating layer 10, and themicroparticle 9, and a second capacitance constituted by the secondcurrent path 2, the second insulating layer 11, and the microparticle 9is 1 nF or less.

FIG. 5B shows that there are a plurality of microparticles 9 between thefirst current path 1 and the second current path 2. In this case, theCoulomb interaction acts between charges existing in the first currentpath 1 and the second current path 2 and charges existing in themicroparticles 9. Due to the Coulomb interaction, the currents flowingthrough the first current path 1 and the second current path 2 and thecharge distribution within the microparticles 9 temporally fluctuate.

In this case, even if the quantum effect of the microparticles 9 is notremarkable, the Coulomb interaction acting between charges of themicroparticles 9 also causes temporal fluctuations of the currentsflowing through the first current path 1 and the second current path 2and the charge distribution within the microparticles 9.

In order to observe fluctuations in charge distribution as a current, itis required that at least one of the capacitance of the microparticle 9and the current paths 1 and 2, and the capacitance of the microparticles9 is 1 nF or less.

The number of the insulating layers between the first current path 1 andthe second current path 2 can be one, or three or more.

FIG. 5C shows the case where air is used as an insulating layer betweenthe first and second current paths and the microparticles 9.

The principle of this is as follows.

The current flowing through either the first current path 1 from a firstsource to a first drain or the second current path 2 from a secondsource to a second drain changes the charge distributions of themicroparticles 9, thereby exerting an influence on the other currentpath. If the current flowing through one current path increases, thecurrent flowing through the other current path is inhibited due to theCoulomb repulsion. Since this phenomenon may temporally change due tothermal fluctuations, etc., it is possible to obtain high-quality randomnumbers if the fluctuations are outputted as “0”s and “1”s.

The intervals of quantum energy levels of the microparticles used in therandom number generating device according to this embodiment should notnecessarily be observed at a predetermined temperature. The fluctuationsin current paths are caused by the temporal fluctuations of the locationof minority charges due to the interactions among the minority chargesor between the minority charges and the current paths.

In this case, however, the degree of the fluctuations of the charges isestimated on the order of the magnitude of charge energy of thecapacitor. Accordingly, the less the capacitance of some portions in themicroparticles 70 is, the greater the effect obtained is.

Third Embodiment

Next, a random number generating device according to the thirdembodiment of the present invention will be described.

In this embodiment, the effect of the strong interaction between thecurrent paths and the charges in the material placed between the currentpaths to fluctuate the current values is used in the case where there isnot a big difference between the plasma frequency inherent to thematerial placed between the current paths and the degree of temporalfluctuations of currents.

FIG. 6A shows the structure of a random number generating deviceaccording to this embodiment.

In this embodiment, there is a structure 13 between the first currentpath 1 and the second current path 2. The structure 13 is either chargedor polarized, and the cycle of the plasma oscillation is about the sameas time Δt_(I) (the above-described formula) in which an electron in thecurrents flowing through the first current path 1 and the second currentpath 2 pass by a region corresponding to the charge distribution in thestructure 13. In this manner, there is a strong interaction between thefluctuations of the charges in the structure 13 and the fluctuations ofthe electrons flowing through the current paths 1 and 2.

Thus, the changes in the charge distribution of the structure 13 and thecurrents flowing through the first current path 1 and the second currentpath 2 become to be in sync with each other, resulting in that thefluctuations of currents occur. Since the fluctuations in the firstcurrent path 1 and those in the second current path 2 are reverse toeach other, it is possible to obtain high-quality random numbers.

FIG. 6A shows that plasma oscillation occurs in the structure 13 betweenthe first current path 1 and the second current path 2 of the randomnumber generating device of this embodiment. The structure 13 includesmicroparticles having a diameter of about 1 μm or less covered by aninsulating layer 12.

Besides this, as shown in FIG. 6B, there is a case where nanoparticles14 having a diameter of about 100 nm or less are located near the firstcurrent path 1 and the second current path 2 and electrically connectedwith each other by a wiring 15. The wiring 15 is electrically connectedwith each nanoparticle 14 via an insulating layer 73. In this case,there are a macroscopic number of electrons between the nanoparticles14, the plasma frequency of which is significantly different from thetemporal fluctuations of the current flowing through the wiring 15.However, if the plasma frequency of the wiring 15 is not significantlydifferent from the frequency of the fluctuations in the first currentpath 1 and the second current path 2, a correlation between the firstcurrent path 1 and the second current path 2 occurs, which can beeffectively used for generating random numbers.

Fourth Embodiment

Next, a random number generating device according to the fourthembodiment of the present invention will be described.

In this embodiment, there are a plurality of trap levels between thecurrent paths. If the degree of the temporal fluctuations of a chargemoving among the trap levels is not significantly different from thedegree of the temporal fluctuations of the currents flowing through thecurrent paths, there is a strong interaction between the current pathsand the charges in a material placed therebetween, thereby fluctuatingcurrent values. This embodiment uses the above-described effect.

As shown in FIG. 7, there is a layer 16 including trap levels 17 betweenthe first current path 1 and the second current path 2. Charges may besequentially transferred from the first current path 1 or the secondcurrent path 2 to the trap levels 17. Alternatively, a considerableamount of charges may be injected at the initial stage.

As a result of an interaction between the first current path 1 and thesecond current path 2 caused by the temporal fluctuations of the chargesof the trap levels 17, high-quality random numbers are generated.

The trap levels can be generated by injecting Ga, B, Si, W, etc., intoan insulating layer by the use of FIB (Focused Ion Beam).

Hereinafter, the structures of the random number generating devicesaccording to the present invention will be specifically described.

FIGS. 8A to 16C show that the random number generating devices of thefirst to the fourth embodiments of the present invention are formed on asilicon substrate. If the size of particles is sufficiently small tocause the quantum effect, the effect explained in the descriptions ofthe first embodiment can be achieved.

If the size of particles is not uniform, and some of the particles arerelatively large, the effect explained in the descriptions of the secondembodiment can also be achieved.

If the size of the particles becomes further larger, the effectexplained in the descriptions of the third embodiment becomes dominant.

If some kinds of trap levels exist in the insulating layer or the like,the effect explained in the descriptions of the fourth embodiment can beachieved. In the random number generating devices shown in FIGS. 13, 14,and 15, the effect of the fourth embodiment can be achieved.

FIRST EXAMPLE

FIG. 8A shows the section view of the random number generating deviceaccording to the first example. A current flows in the directionperpendicular to the sheet of paper.

In the random number generating device, an insulating layer 22 is formedon a silicon substrate 20. Silicon nanoparticles 23 are formed on theinsulating layer 22. The silicon nanoparticles 23 are covered by anoxide layer 24. At the sides of the insulating layer 22 and the oxidelayer 24, two sidewall fine lines 25 are formed.

The two sidewall fine lines 25 serve as the first current path 1 and thesecond current path 2. The silicon nanoparticles 23 are located betweenthe two sidewall fine lines 25. Charges can transfer from/to the twosilicon nanoparticles 23, which constitute the pair of quantum dots 23.

The method of fabricating this device is as follows.

First, as shown in FIG. 8B, device isolation regions 21 are formed onthe silicon substrate 20 to form a device region 51. Then, thermaloxidation of the surface of the silicon substrate 20 is performed toform the silicon oxide layer 22 having a thickness of 10 nm or less. Thesilicon substrate 20 may be of either n-type or p-type.

Next, as shown in FIG. 8C, silicon quantum dots 28 are formed by firstforming a polysilicon layer through CVD, etc., and then annealing thepolysilicon layer.

Then, as shown in FIG. 8D, the oxide layer 24 having a thickness ofabout 8 nm is formed through CVD, etc.

Thereafter, as shown in FIG. 8E, the oxide layer 24 is patterned by theuse of an exposing apparatus. It is preferable that after thepatterning, the remaining portion includes one or more silicon quantumdots 28.

Subsequently, as shown in FIG. 8F, a polysilicon layer is formed throughCVD, etc., and the sidewall fine lines 25 are formed at both the sidesof the portion containing the silicon quantum dots 28 by removing thepolysilicon layer.

Then, as shown in FIG. 8G, masks are formed on the portions separatingthe sources (S) and the drains (D) of the first current path 1 and thesecond current path 2 with a positive photoresist, etc. FIG. 8Gillustrates the device region viewed from the top of the substrate.

Subsequently, as shown in FIG. 8H, ion implantation is performed to formthe sources (S) and the drains (D) serving as contact regions for thecurrents. At this time, if the silicon substrate 20 is of n-type, boron,etc. is used, and if the silicon substrate 20 is of p-type, phosphorusetc., is used. FIG. 8I is a perspective view of FIG. 8H.

Next, an external differential amplifier is connected to the device. Thesidewall fine lines 25 serve as the first current path 1 and the secondcurrent path 2 used to detect the effect of the charge distribution ofthe adjacent quantum dots.

Nanoparticles such as polystyrene beads can be used as the quantum dots28. Further, metal fine particles such as Au can also be used.

Further, instead of the quantum dots 28, an amorphous silicon layer canbe used. Since there are many trap levels in amorphous silicon, it canbe used as a substitute for the pair of quantum dots 23.

In this case, there is no guarantee that pairs of quantum dots areregularly aligned along the current paths. However, if the current pathsare sufficiently longer in view of the size of the quantum dots, it canbe said that the distribution of quantum dots detected by the twocurrent paths is uniform.

FIG. 8J is a section view in the case where STI (Shallow TrenchIsolation) 27 is used for isolating devices.

FIG. 8K is a perspective view in which an SOI substrate 100 is usedinstead of the substrate 200. In the case where the SOI substrate 100 isused, the portion into which ions are injected can reach the oxide layer101 in the substrate.

SECOND EXAMPLE

FIG. 9A is a section view of the random number generating deviceaccording to the second example. Currents flow in the directionperpendicular to the paper plane.

In this random number generating device, an insulating layer 22 isformed on a silicon substrate 20. Silicon microparticles 23 are formedon the insulating layer 22. Under the insulating layer 22 on the siliconsubstrate 20, the first current path 1 and the second current path 2 areformed. The silicon microparticles 23 are arranged between the firstcurrent path 1 and the second current path 2.

The method of fabricating this device is as follows.

First, as shown in FIG. 9B, a device region 51 is formed on the siliconsubstrate 20 so as to have recessed portions. FIG. 9B is a top view. Therecessed portions are not indispensable, since they are for effectivelydetecting the changes in electric field of the quantum dots. Aconduction region is formed by injecting ions to the device region 51.The types of ions to be injected are the same as those for the firstexample.

Next, as shown in FIG. 9C, a trench 30 is formed by exposing the centerportion of the device region by the use of an electron beam exposingapparatus, and etching the silicon substrate 20.

Then, as shown in FIG. 9D, the entire surface is oxidized to form anoxide layer 22. Thereafter, a polysilicon layer is formed on the oxidelayer 22 through CVD, etc., to form silicon quantum dots 28, as in thecase of the first example. FIG. 9G is a perspective view of the devicein FIG. 9D. FIG. 9F is a top view of the device in FIG. 9D.

Subsequently, as shown in FIG. 9E, an oxide layer 80 is depositedthrough CVD, etc., and a contact hole is formed to form a currentterminal. In FIG. 9E, the circled portion has an effect of quantum dotpairs 23.

As in the case of the first example, quantum dots of a material otherthan polysilicon can also be used.

Further, as shown in FIG. 9H, the trench 30 can be formed by anisotropicetching utilizing, e.g., the plane direction of the silicon substrate20.

Further, FIG. 9I is a section view in which STIs 27 are used forisolating devices.

Moreover, FIG. 9J is a section view in which an SOI substrate 100 isused. In the case where the SOI substrate 100 is used, the portion intowhich ions are injected can reach the oxide layer 101 in the substrate.

THIRD EXAMPLE

FIG. 10A is a section view of the random number generating deviceaccording to the third example. Currents flow in the directionperpendicular to the paper plane.

In this random number generating device, silicon nanoparticles 9 arefilled in a trench, and the first current path 1 and the second currentpath 2 are formed at both sides of the trench via an insulating layer31. The first current path 1 and the second current path 2 are formed ona silicon oxide layer 32. A pair of silicon nanoparticles 9 constitutesthe coupled quantum dots 23.

The method of fabricating this device is as follows.

First, as shown in FIG. 10B, a silicon oxide layer 32 having a thicknessof a few hundreds nm or more is formed on a silicon substrate 20. Then,a polysilicon layer 33 is formed on the silicon oxide layer 32. Thepolysilicon layer 33 will serve as the first current path and the secondcurrent path later. Subsequently, a silicon oxide layer 34 is formed onthe polysilicon layer 33 through CVD, etc.

Thereafter, as shown in FIG. 10C, part of the silicon oxide layer 34,the polysilicon layer 33, and the silicon oxide layer 32 is patterned toform a trench 30.

Then, as shown in FIG. 10D, a thin oxide layer 31 is formed on the innersidewall of the trench 30 through thermal oxidation.

Subsequently, as shown in FIG. 10E, silicon quantum dots 9 are formed ofpolysilicon, etc., in the trench 30.

Then, as shown in FIG. 10F, a protection oxide layer 35 is formedthrough CVD, etc.

Subsequently, as shown in FIG. 10G, contacts are formed toward the firstcurrent path and the second current path. FIG. 10G is a top view of thedevice.

FOURTH EXAMPLE

FIG. 11 is a section view of the random number generating deviceaccording to the fourth example. Currents flow in the directionperpendicular to the paper plane.

In this random number generating device, the second current path 2 isformed on an insulating layer 40. The first current path 1 is formed onthe second current path 2 via an insulating layer 41. An insulatinglayer 42 is formed on the first current path 1. The first current path 1and the second current path 2 have end portions formed by etching, atwhich a sidewall insulating layer 43 is formed. Silicon fine particles 9are formed on the sidewall of the sidewall insulating layer 43. Thesesilicon fine particles constitute quantum dots 23.

With such a structure, charges can be transfer within the coupledquantum dots 23. Since the charge distribution has an influence on thefirst current path 1 and the second current path 2, random numbers canbe generated.

The above-described structure can be achieved by patterning part oflaminated layers formed by laminating the insulating layer 40, thesecond current path 2, the insulating layer 41, and the first currentpath 1, oxidizing the angled portion, and depositing the silicon fineparticles 23 on the angled portion.

FIFTH EXAMPLE

FIG. 12 is a section view of a random number generating device accordingto the fifth example.

In this random number generating device, the second current path 2 isformed on a silicon thermal oxide layer 40 obtained by heating thesubstrate. Pairs of quantum dots 23 are formed on the second currentpath 2 by embedding silicon fine particles 9 in a silicon oxide layer 45formed through CVD. The first current path 1 is formed on the pairs ofquantum dots 23. A silicon oxide layer 42 is formed on the first currentpath 1 through CVD.

This structure can be achieved by laminating the silicon thermal oxidelayer 40, the second current path 2, the pairs of quantum dots 23obtained by embedding silicon fine particles 9 in the silicon oxidelayer 45, the first current path 1, and the silicon oxide layer 42, andperforming patterning thereof.

SIXTH EXAMPLE

FIG. 13 is a section view of a random number generating device accordingto the sixth example.

In this random number generating device, the second current path 2 isformed on a silicon thermal oxide layer 50 obtained by heating asubstrate. A silicon oxide layer 51 is formed on the second current path2 through CVD. A silicon nitride layer 52 is formed on the silicon oxidelayer 51. A silicon oxide layer 53 is formed on the silicon nitridelayer 52 through CVD. The first current path 1 is formed on the siliconoxide layer 53. A silicon oxide layer 54 is formed on the first currentpath 1 through CVD.

At the interfaces between the silicon nitride layer 52 and the siliconoxide layer 51, and between the silicon nitride layer 52 and the siliconoxide layer 53, trap levels 55 are formed. The trap levels are formedbetween the first current path 1 and the second current path 2.

SEVENTH EXAMPLE

FIG. 14 is a section view of a random number generating device accordingto the seventh example.

In this random number generating device, the second current path isformed on a silicon thermal oxide layer 50 obtained by hearing asubstrate. A silicon oxide layer 51 is formed on the second current path2 through CVD. An amorphous silicon layer 60 is formed on the siliconoxide layer 51. A silicon oxide layer 53 is formed on the amorphoussilicon layer 60 through CVD. The first current path 1 is formed on thesilicon oxide layer 53. A silicon oxide layer 54 is formed on the firstcurrent path 1 thorough CVD.

The amorphous silicon layer 60 has grain boundaries, part of whichserves as quantum dots. Further, since trap levels exist at grainboundaries of the amorphous silicon layer 60, the trap levels can beused.

EIGHTH EXAMPLE

FIG. 15 is a section view of a random number generating device accordingto the eighth example.

In this random number generating device, pairs of quantum dots 23covered by a thermal oxide layer are formed on a silicon substrate 20. Asilicon layer 61 is formed thereon.

The surface of the silicon substrate 20 serves as the second currentpath 2. The silicon layer 61 serves as the first current path.

This structure can be achieved by laminating silicon on a siliconthermal oxide layer obtained by heating the substrate with the epitaxilatechnique, and then crystallizing it to form the first current path 1.In case the crystallization of the silicon layer 61 is not sufficientlyperformed, a gate electrode 62 is formed thereon. The gate electrode 62is used to adjust the current amount of the first current path 1 and thesecond current path 2.

Hereinafter, other examples will be described.

In FIGS. 16A, 16B, and 16C, the quantum dots in the first, second, andfourth examples are replaced by trap levels 55 formed by injecting ionsof Ga, B, Si, W, and the like by Focused Ion Beam (FIB). The rest of thestructure is the same as the first, second, and fourth examples.

The trap levels may reach the substrate. Further, it is possible to formtrap levels between the silicon substrate and the thermal oxide layer byroughening the surface of the silicon substrate by using KOH or the likewithout using FIB.

In the above-described examples, the first current path and the secondcurrent path are formed of polysilicon. However, instead of polysilicon,a silicide material such as CoSi, FeSi, NiSi, TiSi, etc. can be used inorder to improve the conductivity.

Moreover, the first current path and the second current path can beformed of a metal such as Al, Fe, Ti, Ni, Co, and Cu.

In addition, although a single material is used for the quantum dots ofthe above-described examples, silicidation of the surface of the quantumdots can be performed. Moreover, two types of semiconductors or metalscan be used to form the quantum dots.

Furthermore, in the above-described examples, it is possible to form thegate electrode on the device or on the backside of the substrate so asto adjust the charge distribution of the quantum dots.

Moreover, in order to improve the control of the electron state of thequantum dots, a magnetic field can be applied to the device. Further, inorder to electromagnetically coupling the quantum dots, the quantum dotscan be placed in a cavity or a multiple quantum well structure.

In the above-described examples, pairs of coupled quantum dots aremainly used. However, the number of quantum dots coupled can be three ormore. For example, in the fifth example, the number of laminated quantumdots can be three or more.

Further, the device isolation can be performed by Local Oxidation ofSilicon (LOCOS), Shallow Trench Isolation (STI), or other methods.

Although a silicon oxide layer is used as a tunnel oxide layer or aninsulating layer in the above-described embodiments, a high dielectricconstant material, such as SiN, alumina, HfO₂, ZrO₂, etc. can be usedinstead. Further, two or more of these materials can be laminated.

The first current path and the second current path may have constrictedportions, i.e., so-called the point contact structure. Or, they may besingle electron devices.

Although the substrate is a silicon substrate in the above embodiments,it may be formed of glass, STO, GaN, GaAs, etc.

In the present invention, quantum fluctuations or thermal fluctuationsof subtle charges have influence on currents flowing through currentpaths. Accordingly, it is possible to generate high-quality randomnumbers at a high speed and without a deviation by using the differencein the currents flowing through the currents paths as random numberdata.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcepts as defined by the appended claims and their equivalents.

1. A random number generating device comprising: a semiconductorsubstrate; a pair of first and second current paths formed in thesemiconductor substrate so as to be separated each other; a firstinsulating layer formed on the semiconductor substrate between the firstand second current paths; a pair of first and second fine particleslocated on the first insulating layer and being able to mutuallyexchange charges, at least one of the first and second fine particlesbeing electrically connected to one of the first and second currentpaths; and a random number generator configured to include a detectioncircuit detecting a difference between currents flowing through thefirst current path and the second current path, and generate a randomnumber based on the difference.
 2. The random number generating deviceaccording to claim 1, wherein the first and second fine particles arecovered by a second insulating layer formed on the first insulatinglayer, and a conductive film to connect with the first and secondcurrent paths is formed at side portions of the second insulating layer.3. The random number generating device according to claim 1, furthercomprising a trench formed in the semiconductor surface between thefirst and second current paths, wherein the first insulating layercovers a surface of the trench.
 4. A random number generating devicecomprising: a semiconductor substrate; a first insulating layer formedon the semiconductor substrate; a pair of first and second current pathsformed on the first insulating layer so as to be separated each other; apair of first and second fine particles embedded in a trench formedbetween the first and second current paths, a bottom of the trenchreaching the first insulating layer, and the first and second fineparticles being able to mutually exchange charges, at least one of thefirst and second fine particles being electrically connected to one ofthe first and second current paths; a pair of second insulating layersformed between the first fine particle and the first current path, andbetween the second fine particle and the second current pathrespectively; and a random number generator configured to include adetection circuit detecting a difference between currents flowingthrough the first current path and the second current path, and generatea random number based on the difference.
 5. A random number generatingdevice comprising: a first insulating layer; a first current path formedon the first insulating layer; a second insulating layer formed on thefirst current path; a second current path formed on the secondinsulating layer; a pair of third insulating layers formed at sideportions of the first current path, the second insulating layer and thesecond current path; a pair of first and second fine particles providedon a face of the third insulating layers opposite from the first currentpath, the second insulating layer and the second current path, and thefirst and second fine particles being able to mutually exchange charges,at least one of the first and second fine particles being electricallyconnected to one of the first and second current paths; and a randomnumber generator configured to include a detection circuit detecting adifference between currents flowing through the first current path andthe second current path, and generate a random number based on thedifference.
 6. The random number generating device according to claim 1,wherein the first and second fine particles are formed of Si.
 7. Therandom number generating device according to claim 1, wherein an energylevel of charge including a charging energy in the first and second fineparticles is discrete.
 8. The random number generating device accordingto claim 1, wherein the diameter of the first and second fine particlesis 100 nm or less.
 9. The random number generating device according toclaim 1, wherein an odd number of the pairs of first and second fineparticles are located along the direction of currents flowing throughthe first and second current paths.
 10. The random number generatingdevice according to claim 4, wherein the first and second fine particlesare formed of Si.
 11. The random number generating device according toclaim 4, wherein an energy level of charge including a charging energyin the first and second fine particles is discrete.
 12. The randomnumber generating device according to claim 4, wherein the diameter ofthe first and second fine particles is 100 nm or less.
 13. The randomnumber generating device according to claim 4, wherein an odd number ofthe pairs of first and second fine particles are located along thedirection of currents flowing through the first and second currentpaths.
 14. The random number generating device according to claim 5,wherein the first and second fine particles are formed of Si.
 15. Therandom number generating device according to claim 5, wherein an energylevel of charge including a charging energy in the first and second fineparticles is discrete.
 16. The random number generating device accordingto claim 5, wherein the diameter of the first and second fine particlesis 100 nm or less.
 17. The random number generating device according toclaim 5, wherein an odd number of the pairs of first and second fineparticles are located along the direction of currents flowing throughthe first and second current paths.